kernelwindow.py: Fix callback arguments.

Fixes #12746.

Author: mtwebster

usr/lib/linuxmint/mintUpdate/kernelwindow.py

@@ -239,7 +239,7 @@ def __init__(self, callback=None):
             self.ui_stack.set_visible_child_name("intro_page")
 
     # Refresh window on kernel type selection change
-    def on_kernel_type_combo_changed(widget):
+    def on_kernel_type_combo_changed(self, widget):
         global CONFIGURED_KERNEL_TYPE
         CONFIGURED_KERNEL_TYPE = "-" + widget.get_active_text()
         self.settings.set_string("selected-kernel-type", CONFIGURED_KERNEL_TYPE)